Stacked ESD clamp with reduced variation in clamp voltage

ABSTRACT

An integrated circuit containing a stacked bipolar transistor which includes two bipolar transistors connected in series is disclosed. Each bipolar transistor includes a breakdown inducing feature. The breakdown inducing features have reflection symmetry with respect to each other. A process for forming an integrated circuit containing a stacked bipolar transistor which includes two bipolar transistors connected in series, with breakdown inducing features having reflection symmetry, is also disclosed.

This application claims the benefit of U.S. Provisional Application No.61/394,923, filed Oct. 20, 2010, the entirety of which is hereinincorporated by reference.

FIELD OF THE INVENTION

This invention relates to the field of integrated circuits. Moreparticularly, this invention relates to bipolar transistors inintegrated circuits.

BACKGROUND OF THE INVENTION

Bipolar transistors in integrated circuits may exhibit breakdown betweenbases and collectors of the bipolar transistors when voltages areapplied to the collectors. Breakdown voltages may be sensitive tovariations in locations of breakdown related elements such as collectorsof the bipolar transistors with respect to the bases. Variations in thelocations of the breakdown related elements between instances of thebipolar transistors may be due to alignment variations ofphotolithographic patterns for forming the bases and the breakdownrelated elements during fabrication of the integrated circuit.

SUMMARY OF THE INVENTION

The following presents a simplified summary in order to provide a basicunderstanding of one or more aspects of the invention. This summary isnot an extensive overview of the invention, and is neither intended toidentify key or critical elements of the invention, nor to delineate thescope thereof. Rather, the primary purpose of the summary is to presentsome concepts of the invention in a simplified form as a prelude to amore detailed description that is presented later.

An integrated circuit may contain a stacked bipolar transistor whichincludes two bipolar transistors electrically connected in series. Ineach bipolar transistor, a breakdown inducing feature is located suchthat collector-base breakdown is induced to occur in areas which havereflection symmetry with respect to bases of their respective bipolartransistors. A process of forming the integrated circuit containing thestacked bipolar transistor is also disclosed.

DESCRIPTION OF THE VIEWS OF THE DRAWING

FIG. 1A through FIG. 1E are cross-sections of an integrated circuitcontaining a stacked bipolar transistor formed according to a firstembodiment, depicted in successive stages of fabrication.

FIG. 2 is a cross-section of an integrated circuit containing a stackedbipolar transistor formed according to a second embodiment.

FIG. 3 is a cross-section of an integrated circuit containing a stackedbipolar transistor formed according to a third embodiment.

FIG. 4 is a cross-section of an integrated circuit containing a stackedbipolar transistor formed according to a fourth embodiment.

FIG. 5 is a cross-section of an integrated circuit containing a stackedbipolar transistor formed according to a fifth embodiment.

FIG. 6 is a cross-section of an integrated circuit containing a stackedbipolar transistor formed according to a sixth embodiment.

DETAILED DESCRIPTION

The present invention is described with reference to the attachedfigures, wherein like reference numerals are used throughout the figuresto designate similar or equivalent elements. The figures are not drawnto scale and they are provided merely to illustrate the invention.Several aspects of the invention are described below with reference toexample applications for illustration. It should be understood thatnumerous specific details, relationships, and methods are set forth toprovide an understanding of the invention. One skilled in the relevantart, however, will readily recognize that the invention can be practicedwithout one or more of the specific details or with other methods. Inother instances, well-known structures or operations are not shown indetail to avoid obscuring the invention. The present invention is notlimited by the illustrated ordering of acts or events, as some acts mayoccur in different orders and/or concurrently with other acts or events.Furthermore, not all illustrated acts or events are required toimplement a methodology in accordance with the present invention.

An integrated circuit may contain a stacked bipolar transistor whichincludes two bipolar transistors which have reflection symmetry withrespect to each other, and are electrically connected in series. Eachbipolar transistor contains a breakdown inducing feature which includesa collector and a base of the bipolar transistor, and which may inducecollector-base breakdown in a breakdown region of the transistor. Thebreakdown inducing features and the breakdown regions have reflectionsymmetry with respect to bases of their respective bipolar transistors.Alignment variations during fabrication of the integrated circuit whichmay cause variations in locations of the breakdown inducing featureswith respect to the bases and collectors of their respective bipolartransistors may lead to variations in breakdown voltages of the bipolartransistors. Forming the breakdown inducing features and the breakdownregions to have reflection symmetry with respect to their respectivebases may provide offsetting variations of breakdown voltages betweenthe two bipolar transistors in the stacked bipolar transistor, and somay advantageously provide more uniformity of breakdown voltage amonginstances of the stacked bipolar transistor.

FIG. 1A through FIG. 1E are cross-sections of an integrated circuitcontaining a stacked bipolar transistor formed according to a firstembodiment, depicted in successive stages of fabrication. Referring toFIG. 1A, the integrated circuit 1000 is formed in and on a semiconductorsubstrate 1002. The substrate 1002 may be a single crystal siliconwafer, a silicon-on-insulator (SOI) wafer, a hybrid orientationtechnology (HOT) wafer with regions of different crystal orientations,or other material appropriate for fabrication of the integrated circuit1000. A first buried collector layer 1004 is formed in the substrate1002 in a first bipolar transistor area 1006. A second buried collectorlayer 1008 is formed in the substrate 1002 in a second bipolartransistor area 1010. In the instant embodiment, the substrate 1002 inthe first bipolar transistor area 1006 and in the second bipolartransistor area 1010 is p-type, while the buried collectors 1004 and1008 are n-type. In one realization of the instant embodiment, theburied collectors 1004 and 1008 may be formed by diffusing or ionimplanting n-type dopants such as phosphorus, arsenic or antimony inareas defined for the buried collectors 1004 and 1008 and growingepitaxial semiconductor material over the implanted regions. In analternate realization of the instant embodiment, the buried collectors1004 and 1008 may be formed by ion implanting n-type dopants into thesubstrate 1002 with sufficient energy to form n-type layers at a desireddepth below a top surface of the substrate 1002. Other processes offorming the buried collectors 1004 and 1008 are within the scope of theinstant embodiment.

Referring to FIG. 1B, a first collector right vertical member 1012 isformed in the substrate 1002, extending from a top surface of thesubstrate 1002 to the first buried collector layer 1004. A firstcollector left vertical member 1014 is formed in the substrate 1002,also extending from a top surface of the substrate 1002 to the firstburied collector layer 1004. The first collector left vertical member1014 is laterally separated from the first collector right verticalmember 1012. The first collector right vertical member 1012 and thefirst collector left vertical member 1014 have a same conductivity typeas the first buried collector layer 1004.

A second collector left vertical member 1016 and a second collectorright vertical member 1018 are formed in the substrate 1002, alsoextending from a top surface of the substrate 1002 to the second buriedcollector layer 1008. The second collector left vertical member 1016 andthe second collector right vertical member 1018 are located so as tohave reflection symmetry with respect to the first collector rightvertical member 1012 and the first collector left vertical member 1014,respectively; that is, a lateral direction from the first collectorright vertical member 1012 to the first collector left vertical member1014 is opposite a lateral direction from the second collector leftvertical member 1016 to the second collector right vertical member 1018.A plane of the reflection symmetry is perpendicular to the top surfaceof the substrate 1002 and perpendicular to the plane of FIG. 1B. In theinstant embodiment, a lateral separation between the second collectorleft vertical member 1016 and the second collector right vertical member1018 is substantially equal to a lateral separation between the firstcollector right vertical member 1012 and the first collector leftvertical member 1014.

An optional first well 1020 may be formed between the first collectorright vertical member 1012 and the first collector left vertical member1014, above the first buried collector layer 1004 extending to the topsurface of the substrate 1002. A conductivity type of the first well1020 if formed is opposite from the conductivity type of the firstburied collector layer 1004. An optional second well 1022 may be formedbetween the second collector left vertical member 1016 and the secondcollector right vertical member 1018, above the second buried collectorlayer 1008 extending to the top surface of the substrate 1002. Aconductivity type of the second well 1022 if formed is opposite from theconductivity type of the second buried collector layer 1008.

Referring to FIG. 1C, a first base right diffused region 1024 is formedin the substrate 1002, extending to the top surface of the substrate1002, between the first collector right vertical member 1012 and thefirst collector left vertical member 1014. The first base right diffusedregion 1024 is located closer to the first collector right verticalmember 1012 than to the first collector left vertical member 1014.

A first base left diffused region 1026 is formed in the substrate 1002,extending to the top surface of the substrate 1002, between the firstcollector right vertical member 1012 and the first collector leftvertical member 1014. The first base left diffused region 1026 islocated closer to the first collector left vertical member 1014 than tothe first collector right vertical member 1012. In the instantembodiment, a lateral separation between the first base right diffusedregion 1024 and the first collector right vertical member 1012 is lessthan a lateral separation between the first base left diffused region1026 and the first collector left vertical member 1014. In onerealization of the instant embodiment, the lateral separation betweenthe first base right diffused region 1024 and the first collector rightvertical member 1012 is between 0.75 and 0.85 microns less than thelateral separation between the first base left diffused region 1026 andthe first collector left vertical member 1014.

A second base left diffused region 1028 and a second base right diffusedregion 1030 are formed in the substrate 1002, extending to the topsurface of the substrate 1002, between the second collector leftvertical member 1016 and the second collector right vertical member1018. The second base left diffused region 1028 is located closer to thesecond collector left vertical member 1016 than to the second collectorright vertical member 1018. The second base right diffused region 1030is located closer to the second collector right vertical member 1018than to the second collector left vertical member 1016. In the instantembodiment, a lateral separation between the second base left diffusedregion 1028 and the second collector left vertical member 1016 is lessthan a lateral separation between the second base right diffused region1030 and the second collector right vertical member 1018. in the instantembodiment, the lateral separation between the second base left diffusedregion 1028 and the second collector left vertical member 1016 issubstantially equal to the lateral separation between the first baseright diffused region 1024 and the first collector right vertical member1012.

The base diffused regions 1024, 1026, 1028 and 1030 have a conductivitytype opposite from the collector vertical members 1012, 1014, 1016 and1018. The base diffused regions 1024, 1026, 1028 and 1030 may be formedfor example by ion implanting dopants into the substrate 1002 in areasdefined for the base diffused regions 1024, 1026, 1028 and 1030 by abase diffused region photoresist pattern (not shown), and subsequentlyannealing the substrate 1002.

Referring to FIG. 1D, a first emitter diffused region 1032 is formed inthe substrate 1002 extending to the top surface of the substrate 1002between the first base right diffused region 1024 and the first baseleft diffused region 1026. In the instant embodiment, the first emitterdiffused region 1032 is located substantially equidistant from the firstbase right diffused region 1024 and the first base left diffused region1026.

A second emitter diffused region 1034 is formed in the substrate 1002extending to the top surface of the substrate 1002 between the secondbase left diffused region 1028 and the second base right diffused region1030. In the instant embodiment, the second emitter diffused region 1034is located substantially equidistant from the second base left diffusedregion 1028 and the second base right diffused region 1030.

The emitter diffused regions 1032 and 1034 have a same conductivity typeas the collector vertical members 1012, 1014, 1016 and 1018. The emitterdiffused regions 1032 and 1034 may be formed for example by ionimplanting dopants into the substrate 1002 in areas defined for theemitter diffused regions 1032 and 1034 by an emitter diffused regionphotoresist pattern (not shown), and subsequently annealing thesubstrate 1002. Regions of the substrate 1002 between the emitterdiffused regions 1032 and 1034 and the first bipolar transistor area1006 and the second bipolar transistor area 1010 may be further modifiedby introducing additional dopants, for example by ion implanting dopantsfollowed by one or more subsequent anneal processes.

The first buried collector layer 1004, the first collector rightvertical member 1012 and the first collector left vertical member 1014are included in a collector of the first bipolar transistor. The firstbase right diffused region 1024 and the first base left diffused region1026 are included in a base of the first bipolar transistor. The firstemitter diffused region 1032 is included in an emitter of the firstbipolar transistor. Similarly, the second buried collector layer 1008,the second collector left vertical member 1016 and the second collectorright vertical member 1018 are included in a collector of the secondbipolar transistor. The second base left diffused region 1028 and thesecond base right diffused region 1030 are included in a base of thesecond bipolar transistor. The second emitter diffused region 1034 isincluded in an emitter of the second bipolar transistor.

In the instant embodiment, the first bipolar transistor and the secondbipolar transistor may be electrically coupled in series as depicted inFIG. 1E to form the stacked bipolar transistor. The first collectorright vertical member 1012 and the first collector left vertical member1014 may be coupled to a first circuit node 1036. The first base rightdiffused region 1024 and the first base left diffused region 1026 may becoupled to the first emitter diffused region 1032, possibly throughoptional first resistors 1038. The first emitter diffused region 1032may be coupled to the second collector left vertical member 1016 and thesecond collector right vertical member 1018 for example throughinterconnect elements of the integrated circuit 1000 as depictedschematically by transistor connection 1040. The second base leftdiffused region 1028 and the second base right diffused region 1030 maybe coupled to the second emitter diffused region 1034, possibly throughoptional second resistors 1042. The second emitter diffused region 1034may be coupled to a second circuit node 1044.

During operation of the integrated circuit 1000, a voltage may beapplied to the first circuit node 1036, for example an electro-staticdischarge (ESD) event, which causes collector-base breakdown in thestacked bipolar transistor. Forming the first collector right verticalmember 1012, the first collector left vertical member 1014, the firstbase right diffused region 1024 and the first base left diffused region1026 so that the lateral separation between the first base rightdiffused region 1024 and the first collector right vertical member 1012is less than the lateral separation between the first base left diffusedregion 1026 and the first collector left vertical member 1014 may causecollector-base breakdown in the first bipolar transistor to be inducedin a region between the first base right diffused region 1024 and thefirst collector right vertical member 1012. The configuration of thefirst base right diffused region 1024 and the first collector rightvertical member 1012 is a breakdown inducing feature of the firstbipolar transistor. Similarly, forming the second collector leftvertical member 1016, the second collector right vertical member 1018,the second base left diffused region 1028 and the second base rightdiffused region 1030 so that the lateral separation between the secondbase left diffused region 1028 and the second collector left verticalmember 1016 is less than the lateral separation between the second baseright diffused region 1030 and the second collector right verticalmember 1018 may cause breakdown in the second bipolar transistor to beinduced in a region between the second base left diffused region 1028and the second collector left vertical member 1016. The configuration ofthe second base left diffused region 1028 and the second collector leftvertical member 1016 is a breakdown inducing feature of the secondbipolar transistor. Forming the first bipolar transistor and the secondbipolar transistor as described in reference to FIG. 1A through FIG. 1Dresults in the breakdown inducing feature of the second bipolartransistor having reflection symmetry with respect to the breakdowninducing feature of the first bipolar transistor. A plane of thereflection symmetry is perpendicular to the top surface of the substrate1002 and perpendicular to the planes of FIG. 1A through FIG. 1D.

A first collector-base breakdown voltage of the collector-base breakdownbetween the first base right diffused region 1024 and the firstcollector right vertical member 1012 may be an increasing function ofthe lateral separation between the first base right diffused region 1024and the first collector right vertical member 1012, that is, largerinstances of the lateral separation between the first base rightdiffused region 1024 and the first collector right vertical member 1012provide larger values of the first collector-base breakdown voltage.Instances of the lateral separation between the first base rightdiffused region 1024 and the first collector right vertical member 1012may include unintentional alignment variations, commonly known asmisalignments, in instances of the stacked bipolar transistor.Similarly, a second collector-base breakdown voltage of thecollector-base breakdown between the second base left diffused region1028 and the second collector left vertical member 1016 may be anincreasing function of the lateral separation between the second baseleft diffused region 1028 and the second collector left vertical member1016. The lateral separation between the second base left diffusedregion 1028 and the second collector left vertical member 1016 mayinclude the same misalignment as the lateral separation between thefirst base right diffused region 1024 and the first collector rightvertical member 1012. In such a case, a misalignment which increases thelateral separation between the first base right diffused region 1024 andthe first collector right vertical member 1012 and so increases thefirst collector-base breakdown voltage, may decrease the lateralseparation between the second base left diffused region 1028 and thesecond collector left vertical member 1016 by substantially a samelateral distance and so may decrease the second collector-base breakdownvoltage. A collector-base breakdown voltage of the stacked bipolartransistor is substantially a sum of the first collector-base breakdownvoltage and the second collector-base breakdown voltage. Thus, formingthe breakdown inducing feature of the second bipolar transistor to havereflection symmetry with respect to the breakdown inducing feature ofthe first bipolar transistor may reduce a sensitivity of thecollector-base breakdown voltage of the stacked bipolar transistor tothe misalignments.

It will be recognized that a stacked bipolar transistor of oppositepolarity to that described in reference to FIG. 1A through FIG. 1E maybe formed by appropriate changes in polarities of dopants andconductivity types. A stacked bipolar transistor of opposite polaritymay have a reduced sensitivity of a collector-base breakdown voltage ofthe stacked bipolar transistor to misalignments of collector verticalmembers and base diffused regions, as described in reference to FIG. 1E.

In one realization of the instant embodiment, the first collector leftvertical member 1014 and the second collector right vertical member 1018may be omitted. In one realization of the instant embodiment, the firstbase left diffused region 1026 and the second base right diffused region1030 may be omitted.

FIG. 2 is a cross-section of an integrated circuit containing a stackedbipolar transistor formed according to a second embodiment. Theintegrated circuit 2000 is formed in and on a semiconductor substrate2002, as described in reference to FIG. 1A. An optional first buriedcollector right lobe 2004 and an optional first buried collector leftlobe 2006 may be formed in the substrate 2002 as described in referenceto the first buried collector layer 1004 in FIG. 1A. A first collectorright vertical member 2008 and a first collector left vertical member2010 are formed in the substrate 2002 and extend from a top surface ofthe substrate 2002 to the first buried collector right lobe 2004 and thefirst buried collector left lobe 2006, if present, respectively. Thefirst collector right vertical member 2008 and the first collector leftvertical member 2010 are formed as described in reference to FIG. 1B. Afirst well 2012 is formed between the first collector right verticalmember 2008 and the first collector left vertical member 2010, above thefirst buried collector right lobe 2004 and the first buried collectorleft lobe 2006 if present, extending to the top surface of the substrate2002. A conductivity type of the first well 2012 is opposite from aconductivity type of the first collector right vertical member 2008 andthe first collector left vertical member 2010.

A first base right diffused region 2014 and a first base left diffusedregion 2016 are formed in the substrate 2002 between the first collectorright vertical member 2008 and the first collector left vertical member2010, using processes as described in reference to FIG. 1C. The firstbase right diffused region 2014 is located closer to the first collectorright vertical member 2008 than to the first collector left verticalmember 2010. The first base left diffused region 2016 is located closerto the first collector left vertical member 2010 than to the firstcollector right vertical member 2008. In the instant embodiment, alateral separation between the first base right diffused region 2014 andthe first collector right vertical member 2008 is less than a lateralseparation between the first base left diffused region 2016 and thefirst collector left vertical member 2010. In one realization of theinstant embodiment, the lateral separation between the first base rightdiffused region 2014 and the first collector right vertical member 2008is between 0.75 and 0.85 microns less than the lateral separationbetween the first base left diffused region 2016 and the first collectorleft vertical member 2010.

A first emitter diffused region 2018 is formed in the substrate 2002extending to the top surface of the substrate 2002 between the firstbase right diffused region 2014 and the first base left diffused region2016. In the instant embodiment, the first emitter diffused region 2018is located substantially equidistant from the first base right diffusedregion 2014 and the first base left diffused region 2016. The firstburied collector right lobe 2004, the first buried collector left lobe2006, the first collector right vertical member 2008, the firstcollector left vertical member 2010, the first base right diffusedregion 2014, the first base left diffused region 2016 and the firstemitter diffused region 2018 are included in a first bipolar transistor2020 of the stacked bipolar transistor.

A second bipolar transistor 2022 of the stacked bipolar transistorincludes a second buried collector left lobe 2024 and a second buriedcollector right lobe 2026 if the optional first buried collector lobes2004 and 2006 are present, and includes a second collector left verticalmember 2028, a second collector right vertical member 2030, a secondwell 2032, a second base left diffused region 2034, a second base rightdiffused region 2036 and a second emitter diffused region 2038. Thesecond buried collector left lobe 2024 and the second buried collectorright lobe 2026, if present, are located so as to have reflectionsymmetry with respect to the first buried collector right lobe 2004 andthe first buried collector left lobe 2006, respectively. Similarly, thesecond collector left vertical member 2028 and the second collectorright vertical member 2030 are located so as to have reflection symmetrywith respect to the first collector right vertical member 2008 and thefirst collector left vertical member 2010, respectively. Similarly, thesecond base left diffused region 2034 and the second base right diffusedregion 2036 are located so as to have reflection symmetry with respectto the first base right diffused region 2014 and the first base leftdiffused region 2016, respectively. A plane of the reflection symmetryis perpendicular to the top surface of the substrate 2002 andperpendicular to the plane of FIG. 2. A lateral separation between thesecond base left diffused region 2034 and the second collector leftvertical member 2028 is substantially equal to the lateral separationbetween the first base right diffused region 2014 and the firstcollector right vertical member 2008.

The optional first buried collector right lobe 2004 if present, theoptional first buried collector left lobe 2006 if present, the firstcollector right vertical member 2008 and the first collector leftvertical member 2010 are included in a collector of the first bipolartransistor 2020. The first base right diffused region 2014 and the firstbase left diffused region 2016 are included in a base of the firstbipolar transistor 2020. The first emitter diffused region 2018 isincluded in an emitter of the first bipolar transistor 2020. Similarly,the second buried collector left lobe 2024 if present, the second buriedcollector right lobe 2026 if present, the second collector left verticalmember 2028 and the second collector right vertical member 2030 areincluded in a collector of the second bipolar transistor 2022. Thesecond base left diffused region 2034 and the second base right diffusedregion 2036 are included in a base of the second bipolar transistor2022. The second emitter diffused region 2038 is included in an emitterof the second bipolar transistor 2022.

The first bipolar transistor 2020 and the second bipolar transistor 2022may be electrically coupled in series by interconnect elements 2040 asdescribed in reference to FIG. 1E. The configuration of the first baseright diffused region 2014 and the first collector right vertical member2008 is a breakdown inducing feature of the first bipolar transistor.Similarly, the configuration of the second base left diffused region2034 and the second collector left vertical member 2028 is a breakdowninducing feature of the second bipolar transistor. Forming the breakdowninducing feature of the second bipolar transistor 2022 to havereflection symmetry with respect to the breakdown inducing feature ofthe first bipolar transistor 2020 may reduce a sensitivity of acollector-base breakdown voltage of the stacked bipolar transistor tomisalignments of the collector vertical members 2008, 2010, 2028 and2030 and the base diffused regions 2014, 2016, 2032 and 2034, asdescribed in reference to FIG. 1E. In the instant embodiment, locationsof the first buried collector lobes 2004 and 2006 if present may beadjusted to provide balanced bipolar current density in the firstbipolar transistor 2020 in view of the difference in lateral separationsbetween the first base diffused regions 2014 and 2016 and the firstcollector vertical members 2008 and 2010. Similarly, locations of thesecond buried collector lobes 2024 and 2026 if present may be adjustedto provide balanced bipolar current density in the second bipolartransistor 2022 in view of the difference in lateral separations betweenthe second base diffused regions 2032 and 2034 and the second collectorvertical members 2028 and 2030.

It will be recognized that a stacked bipolar transistor of oppositepolarity to that described in reference to FIG. 2 may be formed byappropriate changes in polarities of dopants and conductivity types. Astacked bipolar transistor of opposite polarity may have a reducedsensitivity of a collector-base breakdown voltage of the stacked bipolartransistor to misalignments of collector vertical members and basediffused regions, as described in reference to FIG. 2.

In one realization of the instant embodiment, the first collector leftvertical member 2010, the first buried collector left lobe 2006, thesecond collector right vertical member 2030 and the second buriedcollector right lobe 2026 may be omitted. In one realization of theinstant embodiment, the first base left diffused region 2016 and thesecond base right diffused region 2036 may be omitted.

FIG. 3 is a cross-section of an integrated circuit containing a stackedbipolar transistor formed according to a third embodiment. Theintegrated circuit 3000 is formed in and on a semiconductor substrate3002, as described in reference to FIG. 1A. An optional first buriedcollector layer 3004 may be formed in the substrate 3002 as described inreference to FIG. 1A. A first collector right vertical member 3006 and afirst collector left vertical member 3008 are formed in the substrate3002 and extend to the first buried collector layer 3004 if present. Thefirst collector right vertical member 3006 and the first collector leftvertical member 3008 are formed as described in reference to FIG. 1B.

A first collector right shallow well 3010 is formed in the substrate3002, extending from a top surface of the substrate 3002 to the firstcollector right vertical member 3006. Similarly, a first collector leftshallow well 3012 is formed in the substrate 3002, extending from thetop surface of the substrate 3002 to the first collector left verticalmember 3008. The first collector right shallow well 3010 and the firstcollector left shallow well 3012 have a same conductivity type as thefirst collector right vertical member 3006. A lateral boundary of thefirst collector right shallow well 3010 on a side closest to the firstcollector left shallow well 3012 extends past a corresponding lateralboundary of the first collector right vertical member 3006. A lateralboundary of the first collector left shallow well 3012 on a side closestto the first collector right shallow well 3010 extends past acorresponding lateral boundary of the first collector left verticalmember 3008. An optional first well 3014 may be formed between the firstcollector right vertical member 3006 and the first collector leftvertical member 3008, above the first buried collector layer 3004 ifpresent and extending to the top surface of the substrate 3002. Aconductivity type of the first well 3014 if formed is opposite from aconductivity type of the first collector right shallow well 3010 and thefirst collector left shallow well 3012.

A first base right diffused region 3016 and a first base left diffusedregion 3018 are formed in the substrate 3002 between the first collectorright shallow well 3010 and the first collector left shallow well 3012,using processes as described in reference to FIG. 1C. The first baseright diffused region 3016 is located closer to the first collectorright shallow well 3010 than to the first collector left shallow well3012. The first base left diffused region 3018 is located closer to thefirst collector left shallow well 3012 than to the first collector rightshallow well 3010. In the instant embodiment, a lateral separationbetween the first base right diffused region 3016 and the firstcollector right shallow well 3010 is less than a lateral separationbetween the first base left diffused region 3018 and the first collectorleft shallow well 3012. In one realization of the instant embodiment,the lateral separation between the first base right diffused region 3016and the first collector right shallow well 3010 is between 0.75 and 0.85microns less than the lateral separation between the first base leftdiffused region 3018 and the first collector left shallow well 3012.

A first emitter diffused region 3020 is formed in the substrate 3002extending to the top surface of the substrate 3002 between the firstbase right diffused region 3016 and the first base left diffused region3018. In the instant embodiment, the first emitter diffused region 3020is located substantially equidistant from the first base right diffusedregion 3016 and the first base left diffused region 3018. The optionalburied collector layer 3004 if present, the first collector rightvertical member 3006, the first collector left vertical member 3008, thefirst collector right shallow well 3010, the first collector leftshallow well 3012, the first base right diffused region 3016, the firstbase left diffused region 3018 and the first emitter diffused region3020 are included in a first bipolar transistor 3022 of the stackedbipolar transistor.

A second bipolar transistor 3024 of the stacked bipolar transistorincludes a second buried collector layer 3026 if the optional firstburied collector layer 3004 is present, and includes a second collectorleft vertical member 3028, a second collector right vertical member3030, a second collector left shallow well 3032, a second collectorright shallow well 3034, an optional second well 3036, a second baseleft diffused region 3038, a second base right diffused region 3040 anda second emitter diffused region 3042. The second collector left shallowwell 3032 and the second collector right shallow well 3034 are locatedso as to have reflection symmetry with respect to the first collectorright shallow well 3010 and the first collector left shallow well 3012,respectively. Similarly, the second base left diffused region 3038 andthe second base right diffused region 3040 are located so as to havereflection symmetry with respect to the first base right diffused region3016 and the first base left diffused region 3018, respectively. A planeof the reflection symmetry is perpendicular to the top surface of thesubstrate 3002 and perpendicular to the plane of FIG. 3. A lateralseparation between the second base left diffused region 3038 and thesecond collector left shallow well 3032 is substantially equal to thelateral separation between the first base right diffused region 3016 andthe first collector right shallow well 3010. The first bipolartransistor 3022 and the second bipolar transistor 3024 may beelectrically coupled in series by interconnect elements 3044 asdescribed in reference to FIG. 1E.

The optional first buried collector layer 3004 if present, the firstcollector right vertical member 3006, the first collector left verticalmember 3008, the first collector right shallow well 3010 and firstcollector left shallow well 3012 are included in a collector of thefirst bipolar transistor 3022. The first base right diffused region 3016and the first base left diffused region 3018 are included in a base ofthe first bipolar transistor 3022. The first emitter diffused region3020 is included in an emitter of the first bipolar transistor 3022.Similarly, the second buried collector layer 3026 if present, the secondcollector left vertical member 3028, the second collector right verticalmember 3030, the second collector left shallow well 3032 and the secondcollector right shallow well 3034 are included in a collector of thesecond bipolar transistor 3024. The second base left diffused region3038 and the second base right diffused region 3040 are included in abase of the second bipolar transistor 3024. The second emitter diffusedregion 3042 is included in an emitter of the second bipolar transistor3024.

Forming the first collector right shallow well 3010, the first collectorleft shallow well 3012, the first base right diffused region 3016 andthe first base left diffused region 3018 so that the lateral separationbetween the first base right diffused region 3016 and the firstcollector right shallow well 3010 is less than the lateral separationbetween the first base left diffused region 3018 and the first collectorleft shallow well 3012 may cause collector-base breakdown in the firstbipolar transistor 3022 to be induced in a region between the first baseright diffused region 3016 and the first collector right shallow well3010. The configuration of the first base right diffused region 3016 andthe first collector right shallow well 3010 is a breakdown inducingfeature of the first bipolar transistor. Similarly, forming the secondcollector left shallow well 3032, the second collector right shallowwell 3034, the second base left diffused region 3038 and the second baseright diffused region 3040 so that the lateral separation between thesecond base left diffused region 3038 and the second collector leftshallow well 3032 is less than the lateral separation between the secondbase right diffused region 3040 and the second collector right shallowwell 3034 may cause breakdown in the second bipolar transistor 3024 tobe induced in a region between the second base left diffused region 3038and the second collector left shallow well 3032. The configuration ofthe second base left diffused region 3038 and the second collector leftshallow well 3032 is a breakdown inducing feature of the second bipolartransistor.

A first collector-base breakdown voltage of the collector-base breakdownbetween the first base right diffused region 3016 and the firstcollector right shallow well 3010 may be an increasing function of thelateral separation between the first base right diffused region 3016 andthe first collector right shallow well 3010. Instances of the lateralseparation between the first base right diffused region 3016 and thefirst collector right shallow well 3010 may include misalignments ininstances of the stacked bipolar transistor. Similarly, a secondcollector-base breakdown voltage of the collector-base breakdown betweenthe second base left diffused region 3038 and the second collector leftshallow well 3032 may be an increasing function of the lateralseparation between the second base left diffused region 3038 and thesecond collector left shallow well 3032. The lateral separation betweenthe second base left diffused region 3038 and the second collector leftshallow well 3032 may include the same misalignment as the lateralseparation between the first base right diffused region 3016 and thefirst collector right shallow well 3010. In such a case, a misalignmentwhich increases the lateral separation between the first base rightdiffused region 3016 and the first collector right shallow well 3010 andso increases the first collector-base breakdown voltage, may decreasethe lateral separation between the second base left diffused region 3038and the second collector left shallow well 3032 by substantially a samelateral distance and so may decrease the second collector-base breakdownvoltage. A collector-base breakdown voltage of the stacked bipolartransistor is substantially a sum of the first collector-base breakdownvoltage and the second collector-base breakdown voltage. Thus, formingthe breakdown inducing feature of the second bipolar transistor 3024 tohave reflection symmetry with respect to the breakdown inducing featureof the first bipolar transistor 3022 may reduce a sensitivity of thecollector-base breakdown voltage of the stacked bipolar transistor tothe misalignments.

It will be recognized that a stacked bipolar transistor of oppositepolarity to that described in reference to FIG. 3 may be formed byappropriate changes in polarities of dopants and conductivity types. Astacked bipolar transistor of opposite polarity may have a reducedsensitivity of a collector-base breakdown voltage of the stacked bipolartransistor to misalignments of collector shallow wells and base diffusedregions, as described in reference to FIG. 3.

In one realization of the instant embodiment, the first collector leftvertical member 3008, the first collector left shallow well 3012, thesecond collector right vertical member 3030 and the second collectorright shallow well 3034 may be omitted. In one realization of theinstant embodiment, the first base left diffused region 3018 and thesecond base right diffused region 3040 may be omitted.

FIG. 4 is a cross-section of an integrated circuit containing a stackedbipolar transistor formed according to a fourth embodiment. Theintegrated circuit 4000 is formed in and on a semiconductor substrate4002, as described in reference to FIG. 1A. An optional first buriedcollector layer 4004 may be formed in the substrate 4002 as described inreference to FIG. 1A. A first collector right vertical member 4006 and afirst collector left vertical member 4008 are formed in the substrate4002 and extend from a top surface of the substrate 4002 to the firstburied collector layer 4004 if present. The first collector rightvertical member 4006 and the first collector left vertical member 4008are formed as described in reference to FIG. 1B. An optional first well4010 may be formed between the first collector right vertical member4006 and the first collector left vertical member 4008, above the firstburied collector layer 4004 and extending to the top surface of thesubstrate 4002. A conductivity type of the first well 4010 if formed isopposite from a conductivity type of the first collector right verticalmember 4006 and the first collector left vertical member 4008.

A first base right diffused region 4012 and a first base left diffusedregion 4014 are formed in the substrate 4002 between the first collectorright vertical member 4006 and the first collector left vertical member4008, using processes as described in reference to FIG. 1C. The firstbase right diffused region 4012 is located closer to the first collectorright vertical member 4006 than to the first collector left verticalmember 4008. The first base left diffused region 4014 is located closerto the first collector left vertical member 4008 than to the firstcollector right vertical member 4006. In the instant embodiment, alateral separation between the first base right diffused region 4012 andthe first collector right vertical member 4006 is substantially equal toa lateral separation between the first base left diffused region 4014and the first collector left vertical member 4008.

A first emitter diffused region 4016 is formed in the substrate 4002extending to the top surface of the substrate 4002 between the firstbase right diffused region 4012 and the first base left diffused region4014. In the instant embodiment, the first emitter diffused region 4016is located substantially equidistant from the first base right diffusedregion 4012 and the first base left diffused region 4014.

A first transistor right field plate 4018 is formed over the substrate4002 between the first base right diffused region 4012 and the firstcollector right vertical member 4006. A first transistor left fieldplate 4020 is formed over the substrate 4002 between the first base leftdiffused region 4014 and the first collector left vertical member 4008.The first transistor field plates 4018 and 4020 may be formed ofpolycrystalline silicon, commonly known as polysilicon, a combination ofpolysilicon and metal silicide, a metal such as tungsten, titaniumnitride, aluminum or copper, or other electrically conductive material.The first transistor field plates 4018 and 4020 may be formed over adielectric layer to provide electrical isolation from the substrate4002.

The optional first buried collector layer 4004 if present, the firstcollector right vertical member 4006, the first collector left verticalmember 4008, the first base right diffused region 4012, the first baseleft diffused region 4014, the first emitter diffused region 4016, thefirst transistor right field plate 4018 and the first transistor leftfield plate 4020 are included in a first bipolar transistor 4022 of thestacked bipolar transistor. The optional first buried collector layer4004 if present, the first collector right vertical member 4006 and thefirst collector left vertical member 4008 are included in a collector ofthe first bipolar transistor 4022. The first base right diffused region4012 and the first base left diffused region 4014 are included in a baseof the first bipolar transistor 4022. The first emitter diffused region4016 is included in an emitter of the first bipolar transistor 4022.

The first transistor right field plate 4018 and the first transistorleft field plate 4020 are formed so that collector-base breakdown in thefirst bipolar transistor 4022 may be induced under the first transistorright field plate 4018. The configuration of the first collector rightvertical member 4006, the first base right diffused region 4012 and thefirst transistor right field plate 4018 is a breakdown inducing featureof the first bipolar transistor. In one realization of the instantembodiment, a length of the first transistor right field plate 4018 maybe greater than a length of the first transistor left field plate 4020.In one realization of the instant embodiment, a distance of the firsttransistor right field plate 4018 from the first collector rightvertical member 4006 may be less than a distance of the first transistorleft field plate 4020 from the first collector left vertical member4008.

A second bipolar transistor 4024 of the stacked bipolar transistorincludes a second buried collector layer 4026 if the optional firstburied collector layer 4004 is present, and includes a second collectorleft vertical member 4028, a second collector right vertical member4030, an optional second well 4032, a second base left diffused region4034, a second base right diffused region 4036, a second emitterdiffused region 4038, a second transistor left field plate 4040 and asecond transistor right field plate 4042.

The second buried collector layer 4026 if present, second collector leftvertical member 4028 and the second collector right vertical member 4030are included in a collector of the second bipolar transistor 4024. Thesecond base left diffused region 4034 and the second base right diffusedregion 4036 are included in a base of the second bipolar transistor4024. The second emitter diffused region 4038 is included in an emitterof the second bipolar transistor 4024.

The second transistor left field plate 4040 and the second transistorright field plate 4042 are located so as to have reflection symmetrywith respect to the first transistor right field plate 4018 and thefirst transistor left field plate 4020, respectively. Similarly, thesecond base left diffused region 4034 and the second base right diffusedregion 4036 are located so as to have reflection symmetry with respectto the first base right diffused region 4012 and the first base leftdiffused region 4014, respectively. A spatial configuration of thesecond base left diffused region 4034, the second transistor left fieldplate 4040 and the second collector left vertical member 4028 hasreflection symmetry to a spatial configuration of the first base rightdiffused region 4012, the first transistor right field plate 4018 andthe first collector right vertical member 4006. A plane of thereflection symmetry is perpendicular to the top surface of the substrate4002 and perpendicular to the plane of FIG. 4. Forming the second baseleft diffused region 4034, the second transistor left field plate 4040and the second collector left vertical member 4028 to have a spatialconfiguration with reflection symmetry of the spatial configuration ofthe first base right diffused region 4012, the first transistor rightfield plate 4018 and the first collector right vertical member 4006 mayinduce collector-base breakdown in the second bipolar transistor 4024under the second transistor left field plate 4040. The configuration ofthe second collector left vertical member 4028, the second base leftdiffused region 4034 and the second transistor left field plate 4040 isa breakdown inducing feature of the second bipolar transistor. The firstbipolar transistor 4022 and the second bipolar transistor 4024 may beelectrically coupled in series by interconnect elements 4044 asdescribed in reference to FIG. 1E.

A first collector-base breakdown voltage of the collector-base breakdownbetween the first base right diffused region 4012 and the firstcollector right vertical member 4006 may be a function of the spatialconfiguration of the first base right diffused region 4012, the firsttransistor right field plate 4018 and the first collector right verticalmember 4006. Instances of the spatial configuration of the first baseright diffused region 4012, the first transistor right field plate 4018and the first collector right vertical member 4006 may includemisalignments in instances of the stacked bipolar transistor. Similarly,a second collector-base breakdown voltage of the collector-basebreakdown between the second base left diffused region 4034 and thesecond collector left vertical member 4028 may be a function of thespatial configuration of the second base left diffused region 4034, thesecond transistor left field plate 4040 and the second collector leftvertical member 4028, such that instances of the misalignments whichincrease the first collector-base breakdown voltage may decrease thesecond collector-base breakdown voltage, and vice versa. Acollector-base breakdown voltage of the stacked bipolar transistor issubstantially a sum of the first collector-base breakdown voltage andthe second collector-base breakdown voltage. Thus, forming the breakdowninducing feature of the second bipolar transistor 4024 to havereflection symmetry with respect to the breakdown inducing feature ofthe first bipolar transistor 4022 may reduce a sensitivity of thecollector-base breakdown voltage of the stacked bipolar transistor tothe misalignments.

It will be recognized that a stacked bipolar transistor of oppositepolarity to that described in reference to FIG. 4 may be formed byappropriate changes in polarities of dopants and conductivity types. Astacked bipolar transistor of opposite polarity may have a reducedsensitivity of a collector-base breakdown voltage of the stacked bipolartransistor to misalignments, as described in reference to FIG. 4.

In one realization of the instant embodiment, the first collector leftvertical member 4008 and the second collector right vertical member 4030may be omitted. In one realization of the instant embodiment, the firsttransistor left field plate 4020 and the second transistor right fieldplate 4042 may be omitted. In one realization of the instant embodiment,the first base left diffused region 4014 and the second base rightdiffused region 4036 may be omitted.

FIG. 5 is a cross-section of an integrated circuit containing a stackedbipolar transistor formed according to a fifth embodiment. Theintegrated circuit 5000 is formed in and on a semiconductor substrate5002, as described in reference to FIG. 1A. A first buried collectorright lobe 5004 and a first buried collector left lobe 5006 are formedin the substrate 5002 as described in reference to the first buriedcollector layer 1004 in FIG. 1A. A first collector right vertical member5008 and a first collector left vertical member 5010 are formed in thesubstrate 5002 and extend from a top surface of the substrate 5002 tothe first buried collector right lobe 5004 and the first buriedcollector left lobe 5006, respectively. The first collector rightvertical member 5008 and the first collector left vertical member 5010are formed as described in reference to FIG. 1B. A first well 5012 isformed between the first collector right vertical member 5008 and thefirst collector left vertical member 5010, above the first buriedcollector right lobe 5004 and the first buried collector left lobe 5006,extending to the top surface of the substrate 5002. A conductivity typeof the first well 5012 is opposite a conductivity type of the firstburied collector right lobe 5004 and the first buried collector leftlobe 5006.

A first base right diffused region 5014 and a first base left diffusedregion 5016 are formed in the substrate 5002 between the first collectorright vertical member 5008 and the first collector left vertical member5010, using processes as described in reference to FIG. 1C. The firstbase right diffused region 5014 is located closer to the first collectorright vertical member 5008 than to the first collector left verticalmember 5010. The first base left diffused region 5016 is located closerto the first collector left vertical member 5010 than to the firstcollector right vertical member 5008.

A first emitter diffused region 5018 is formed in the substrate 5002extending to the top surface of the substrate 5002 between the firstbase right diffused region 5014 and the first base left diffused region5016. In the instant embodiment, the first emitter diffused region 5018is located substantially equidistant from the first base right diffusedregion 5014 and the first base left diffused region 5016. The firstburied collector right lobe 5004, the first buried collector left lobe5006, the first collector right vertical member 5008, the firstcollector left vertical member 5010, the first base right diffusedregion 5014, the first base left diffused region 5016 and the firstemitter diffused region 5018 are included in a first bipolar transistor5020 of the stacked bipolar transistor. The first buried collector rightlobe 5004, the first buried collector left lobe 5006, the firstcollector right vertical member 5008 and the first collector leftvertical member 5010 are included in a collector of the first bipolartransistor 5020. The first base right diffused region 5014 and the firstbase left diffused region 5016 are included in a base of the firstbipolar transistor 5020. The first emitter diffused region 5018 isincluded in an emitter of the first bipolar transistor 5020. In theinstant embodiment, an underlap of the first buried collector right lobe5004 under the first base right diffused region 5014 is more than anunderlap of the first buried collector left lobe 5006 under the firstbase left diffused region 5016.

A second bipolar transistor 5022 of the stacked bipolar transistorincludes a second buried collector left lobe 5024, a second buriedcollector right lobe 5026, a second collector left vertical member 5028,a second collector right vertical member 5030, a second well 5032, asecond base left diffused region 5034, a second base right diffusedregion 5036 and a second emitter diffused region 5038. The second buriedcollector left lobe 5024 and the second buried collector right lobe5026, are formed so as to have reflection symmetry with respect to thefirst buried collector right lobe 5004 and the first buried collectorleft lobe 5006, respectively. Similarly, the second collector leftvertical member 5028 and the second collector right vertical member 5030are located so as to have reflection symmetry with respect to the firstcollector right vertical member 5008 and the first collector leftvertical member 5010, respectively. Similarly, the second base leftdiffused region 5034 and the second base right diffused region 5036 arelocated so as to have reflection symmetry with respect to the first baseright diffused region 5014 and the first base left diffused region 5016,respectively. A plane of the reflection symmetry is perpendicular to thetop surface of the substrate 5002 and perpendicular to the plane of FIG.2. An underlap of the second buried collector left lobe 5024 under thesecond base left diffused region 5034 is substantially equal to theunderlap of the first buried collector right lobe 5004 under the firstbase right diffused region 5014.

The second buried collector left lobe 5024, the second buried collectorright lobe 5026, the second collector left vertical member 5028 and thesecond collector right vertical member 5030 are included in a collectorof the second bipolar transistor 5022. The second base left diffusedregion 5034 and the second base right diffused region 5036 are includedin a base of the second bipolar transistor 5022. The second emitterdiffused region 5038 is included in an emitter of the second bipolartransistor 5022. The first bipolar transistor 5020 and the secondbipolar transistor 5022 may be electrically coupled in series byinterconnect elements 5040 as described in reference to FIG. 1E.

Forming the first buried collector right lobe 5004, the first buriedcollector left lobe 5006, the first base right diffused region 5014 andthe first base left diffused region 5016 so that the underlap of thefirst buried collector right lobe 5004 under the first base rightdiffused region 5014 is greater than the underlap of the first buriedcollector left lobe 5006 under the first base left diffused region 5016may cause collector-base breakdown in the first bipolar transistor 5020to be induced in a region between the first base right diffused region5014 and the first buried collector right lobe 5004. The configurationof the first buried collector right lobe 5004 and the first base rightdiffused region 5014 is a breakdown inducing feature of the firstbipolar transistor. Similarly, forming the second buried collector leftlobe 5024, the second buried collector right lobe 5026, the second baseleft diffused region 5034 and the second base right diffused region 5036so that the underlap of the second buried collector left lobe 5024 underthe second base left diffused region 5034 is greater than the underlapof the second buried collector right lobe 5026 under the second baseright diffused region 5036 may cause breakdown in the second bipolartransistor 5022 to be induced in a region between the second base leftdiffused region 5034 and the second buried collector left lobe 5024. Theconfiguration of the second base left diffused region 5034 and thesecond buried collector left lobe 5024 is a breakdown inducing featureof the second bipolar transistor.

A first collector-base breakdown voltage of the collector-base breakdownbetween the first base right diffused region 5014 and the first buriedcollector right lobe 5004 may be an increasing function of the underlapof the first buried collector right lobe 5004 under the first base rightdiffused region 5014. Instances of the underlap of the first buriedcollector right lobe 5004 under the first base right diffused region5014 may include misalignments in instances of the stacked bipolartransistor. Similarly, a second collector-base breakdown voltage of thecollector-base breakdown between the second base left diffused region5034 and the second buried collector left lobe 5024 may be an increasingfunction of the underlap of the second buried collector left lobe 5024under the second base left diffused region 5034. The underlap of thesecond buried collector left lobe 5024 under the second base leftdiffused region 5034 may include the same misalignment as the underlapof the first buried collector right lobe 5004 under the first base rightdiffused region 5014. In such a case, a misalignment which increases theunderlap of the first buried collector right lobe 5004 under the firstbase right diffused region 5014 and so increases the firstcollector-base breakdown voltage, may decrease the underlap of thesecond buried collector left lobe 5024 under the second base leftdiffused region 5034 by substantially a same distance and so maydecrease the second collector-base breakdown voltage. A collector-basebreakdown voltage of the stacked bipolar transistor is substantially asum of the first collector-base breakdown voltage and the secondcollector-base breakdown voltage. Thus, forming the breakdown inducingfeature of the second bipolar transistor 5022 to have reflectionsymmetry with respect to the breakdown inducing feature of the firstbipolar transistor 5020 may reduce a sensitivity of the collector-basebreakdown voltage of the stacked bipolar transistor to themisalignments.

It will be recognized that a stacked bipolar transistor of oppositepolarity to that described in reference to FIG. 5 may be formed byappropriate changes in polarities of dopants and conductivity types. Astacked bipolar transistor of opposite polarity may have a reducedsensitivity of a collector-base breakdown voltage of the stacked bipolartransistor to misalignments, as described in reference to FIG. 5.

In one realization of the instant embodiment, the first collector leftvertical member 5010 and the second collector right vertical member 5030may be omitted. In one realization of the instant embodiment, the firstburied collector left lobe 5006 and the second buried collector rightlobe 5026 may be omitted. In one realization of the instant embodiment,the first base left diffused region 5016 and the second base rightdiffused region 5036 may be omitted.

FIG. 6 is a cross-section of an integrated circuit containing a stackedbipolar transistor formed according to a sixth embodiment. Theintegrated circuit 6000 is formed in and on a semiconductor substrate6002, as described in reference to FIG. 1A. An optional first buriedcollector layer 6004 may be formed in the substrate 6002 as described inreference to FIG. 1A. A first collector right vertical member 6006 and afirst collector left vertical member 6008 are formed in the substrate6002 and extend from a top surface of the substrate 6002 to the firstburied collector layer 6004 if present. The first collector rightvertical member 6006 and the first collector left vertical member 6008are formed as described in reference to FIG. 1B.

A first base right diffused region 6010 and a first base left diffusedregion 6012 are formed in the substrate 6002 between the first collectorright vertical member 6006 and the first collector left vertical member6008, using processes as described in reference to FIG. 1C. The firstbase right diffused region 6010 is located closer to the first collectorright vertical member 6006 than to the first collector left verticalmember 6008. The first base left diffused region 6012 is located closerto the first collector left vertical member 6008 than to the firstcollector right vertical member 6006. In the instant embodiment, alateral separation between the first base right diffused region 6010 andthe first collector right vertical member 6006 is substantially equal toa lateral separation between the first base left diffused region 6012and the first collector left vertical member 6008.

A first emitter diffused region 6014 is formed in the substrate 6002extending to the top surface of the substrate 6002 between the firstbase right diffused region 6010 and the first base left diffused region6012. In the instant embodiment, the first emitter diffused region 6014is located substantially equidistant from the first base right diffusedregion 6010 and the first base left diffused region 6012.

A first transistor right implanted region 6016 having a sameconductivity type as the first base right diffused region 6010 is formedin the substrate 6002 between the first base right diffused region 6010and the first collector right vertical member 6006. A first transistorleft implanted region 6018 having the same conductivity type as thefirst base left diffused region 6012 is formed in the substrate 6002between the first base left diffused region 6012 and the first collectorleft vertical member 6008. The first transistor implanted regions 6016and 6018 may be formed by ion implanting dopants followed by an annealprocess.

The optional first buried collector layer 6004 if present, the firstcollector right vertical member 6006, the first collector left verticalmember 6008, the first base right diffused region 6010, the first baseleft diffused region 6012, the first emitter diffused region 6014, thefirst transistor right implanted region 6016 and the first transistorleft implanted region 6018 are included in a first bipolar transistor6020 of the stacked bipolar transistor. The optional first buriedcollector layer 6004 if present, the first collector right verticalmember 6006 and the first collector left vertical member 6008 areincluded in a collector of the first bipolar transistor 6020. The firstbase right diffused region 6010 and the first base left diffused region6012 are included in a base of the first bipolar transistor 6020. Thefirst emitter diffused region 6014 is included in an emitter of thefirst bipolar transistor 6020.

The first transistor right implanted region 6016 and the firsttransistor left implanted region 6018 are formed so that collector-basebreakdown in the first bipolar transistor 6020 may be induced betweenthe first collector right vertical member 6006 and the first base rightdiffused region 6010. The configuration of the first base right diffusedregion 6010, first transistor right implanted region 6016 and the firstcollector right vertical member 6006 is a breakdown inducing feature ofthe first bipolar transistor. In one realization of the instantembodiment, a length of the first transistor right implanted region 6016may be greater than a length of the first transistor left implantedregion 6018. In one realization of the instant embodiment, a dopingdensity of the first transistor right implanted region 6016 may begreater than a doping density of the first transistor left implantedregion 6018.

A second bipolar transistor 6022 of the stacked bipolar transistorincludes a second buried collector layer 6024 if the optional firstburied collector layer 6004 is present, and includes a second collectorleft vertical member 6026, a second collector right vertical member6028, a second base left diffused region 6030, a second base rightdiffused region 6032, a second emitter diffused region 6034, a secondtransistor left implanted region 6036 having the same conductivity typeas the second base left diffused region 6030 and a second transistorright implanted region 6038 having the same conductivity type as thesecond base right diffused region 6032. The second buried collectorlayer 6024 if present, the second collector left vertical member 6026and the second collector right vertical member 6028 are included in acollector of the second bipolar transistor 6022. The second base leftdiffused region 6030 and the second base right diffused region 6032 areincluded in a base of the second bipolar transistor 6022. The secondemitter diffused region 6034 is included in an emitter of the secondbipolar transistor 6022. The second transistor left implanted region6036 and the second transistor right implanted region 6038 are locatedso as to have reflection symmetry with respect to the first transistorright implanted region 6016 and the first transistor left implantedregion 6018, respectively. Similarly, the second base left diffusedregion 6030 and the second base right diffused region 6032 are locatedso as to have reflection symmetry with respect to the first base rightdiffused region 6010 and the first base left diffused region 6012,respectively. A spatial configuration of the second base left diffusedregion 6030, the second transistor left implanted region 6036 and thesecond collector left vertical member 6026 has reflection symmetry to aspatial configuration of the first base right diffused region 6010, thefirst transistor right implanted region 6016 and the first collectorright vertical member 6006. A plane of the reflection symmetry isperpendicular to the top surface of the substrate 6002 and perpendicularto the plane of FIG. 6. Forming the second base left diffused region6030, the second transistor left implanted region 6036 and the secondcollector left vertical member 6026 to have a spatial configuration withreflection symmetry of the spatial configuration of the first base rightdiffused region 6010, the first transistor right implanted region 6016and the first collector right vertical member 6006 may inducecollector-base breakdown in the second bipolar transistor 6022 under thesecond transistor left implanted region 6036. The configuration of thesecond base left diffused region 6030, second transistor left implantedregion 6036 and the second collector left vertical member 6026 is abreakdown inducing feature of the second bipolar transistor. The firstbipolar transistor 6020 and the second bipolar transistor 6022 may beelectrically coupled in series by interconnect elements 6040 asdescribed in reference to FIG. 1E.

A first collector-base breakdown voltage of the collector-base breakdownbetween the first base right diffused region 6010 and the firstcollector right vertical member 6006 may be a function of the spatialconfiguration of the first base right diffused region 6010, the firsttransistor right implanted region 6016 and the first collector rightvertical member 6006. Instances of the spatial configuration of thefirst base right diffused region 6010, the first transistor rightimplanted region 6016 and the first collector right vertical member 6006may include misalignments in instances of the stacked bipolartransistor. Similarly, a second collector-base breakdown voltage of thecollector-base breakdown between the second base left diffused region6030 and the second collector left vertical member 6026 may be afunction of the spatial configuration of the second base left diffusedregion 6030, the second transistor left implanted region 6036 and thesecond collector left vertical member 6026, such that instances of themisalignments which increase the first collector-base breakdown voltagemay decrease the second collector-base breakdown voltage, and viceversa. A collector-base breakdown voltage of the stacked bipolartransistor is substantially a sum of the first collector-base breakdownvoltage and the second collector-base breakdown voltage. Thus, formingthe breakdown inducing feature of the second bipolar transistor 6022 tohave reflection symmetry with respect to the breakdown inducing featureof the first bipolar transistor 6020 may reduce a sensitivity of thecollector-base breakdown voltage of the stacked bipolar transistor tothe misalignments.

It will be recognized that a stacked bipolar transistor of oppositepolarity to that described in reference to FIG. 6 may be formed byappropriate changes in polarities of dopants and conductivity types. Astacked bipolar transistor of opposite polarity may have a reducedsensitivity of a collector-base breakdown voltage of the stacked bipolartransistor to misalignments, as described in reference to FIG. 6.

In one realization of the instant embodiment, the first collector leftvertical member 6008 and the second collector right vertical member 6028may be omitted. In one realization of the instant embodiment, the firsttransistor left implanted region 6018 and the second transistor rightimplanted region 6038 may be omitted. In one realization of the instantembodiment, the first base left diffused region 6012 and the second baseright diffused region 6032 may be omitted.

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only and not limitation. Numerous changes to the disclosedembodiments can be made in accordance with the disclosure herein withoutdeparting from the spirit or scope of the invention. Thus, the breadthand scope of the present invention should not be limited by any of theabove described embodiments. Rather, the scope of the invention shouldbe defined in accordance with the following claims and theirequivalents.

What is claimed is:
 1. A process of forming an integrated circuit,comprising the steps: providing a semiconductor substrate; forming astacked bipolar transistor, by a process including the steps: forming afirst bipolar transistor, by a process further including the steps:forming a collector in said substrate, such that said collector of saidfirst bipolar transistor has a first conductivity type; forming a basein said substrate proximate to said collector of said first bipolartransistor, such that said base of said first bipolar transistor isseparated from said collector of said first bipolar transistor by aregion of said substrate, and such that said base of said first bipolartransistor has a conductivity type opposite from said collector of saidfirst bipolar transistor; forming an emitter in said substrate proximateto said base of said first bipolar transistor opposite from saidcollector of said first bipolar transistor, such that said emitter ofsaid first bipolar transistor is separated from said base of said firstbipolar transistor by a region of said substrate, and such that saidemitter of said first bipolar transistor has a same conductivity type assaid collector of said first bipolar transistor; and forming a breakdowninducing feature, such that said breakdown inducing feature of saidfirst bipolar transistor includes said collector of said first bipolartransistor and said base of said first bipolar transistor; forming asecond bipolar transistor, by a process further including the steps:forming a collector in said substrate, such that said collector of saidsecond bipolar transistor has a same conductivity type as said collectorof said first bipolar transistor; forming a base in said substrateproximate to said collector of said second bipolar transistor, such thatsaid base of said second bipolar transistor is separated from saidcollector of said second bipolar transistor by a region of saidsubstrate, and such that said base of said second bipolar transistor hasa same conductivity type as said base of said first bipolar transistor;forming an emitter in said substrate proximate to said base of saidsecond bipolar transistor opposite from said collector of said secondbipolar transistor, such that said emitter of said second bipolartransistor is separated from said base of said second bipolar transistorby a region of said substrate, and such that said emitter of said secondbipolar transistor has a same conductivity type as said collector ofsaid second bipolar transistor; and forming a breakdown inducingfeature, such that said breakdown inducing feature of said secondbipolar transistor includes said collector of said second bipolartransistor and said base of said second bipolar transistor, and suchthat said breakdown inducing feature of said second bipolar transistorhas reflection symmetry with respect to said breakdown inducing featureof said first bipolar transistor, in which a plane of said reflectionsymmetry is perpendicular to a top surface of said substrate; andforming interconnect elements, such that: said collector of said firstbipolar transistor is coupled to a first circuit node; said emitter ofsaid first bipolar transistor is coupled to said collector of saidsecond bipolar transistor; and said emitter of said second bipolartransistor is coupled to a second circuit node.
 2. The process of claim1, in which: said step of forming said collector of said first bipolartransistor further includes the step of forming a first collector rightvertical member in said substrate and proximate to said emitter of saidfirst bipolar transistor; said step of forming said base of said firstbipolar transistor further includes the step of forming a first baseright diffused region in said substrate between said emitter of saidfirst bipolar transistor and said first collector right vertical member;said step of forming said breakdown inducing feature of said firstbipolar transistor is performed such that said breakdown inducingfeature of said first bipolar transistor further includes a region ofsaid substrate between said first collector right vertical member andsaid first base right diffused region; said step of forming saidcollector of said second bipolar transistor further includes the step offorming a second collector left vertical member in said substrate andproximate to said emitter of said second bipolar transistor; said stepof forming said base of said second bipolar transistor further includesthe step of forming a second base left diffused region in said substratebetween said emitter of said second bipolar transistor and said secondcollector left vertical member; and said step of forming said breakdowninducing feature of said second bipolar transistor is performed suchthat said breakdown inducing feature of said second bipolar transistorfurther includes a region of said substrate between said secondcollector left vertical member and said second base left diffusedregion, and such that a configuration of said second collector leftvertical member and said second base left diffused region has saidreflection symmetry with respect to a configurations of said firstcollector right vertical member and said first base right diffusedregion.
 3. The process of claim 2, in which: said process of formingsaid collector of said first bipolar transistor further includes thestep of forming a first buried collector layer in said substrate, suchthat a bottom surface of said first collector right vertical membercontacts a top surface of said first buried collector layer; and saidprocess of forming said collector of said second bipolar transistorfurther includes the step of forming a second buried collector layer insaid substrate, such that a bottom surface of said second collector leftvertical member contacts a top surface of said second buried collectorlayer.
 4. The process of claim 2, in which: said process of forming saidbase of said first bipolar transistor further includes the step offorming a first base left diffused region in said substrate proximate tosaid emitter of said first bipolar transistor and opposite from saidfirst base right diffused region; said process of forming said collectorof said first bipolar transistor further includes the steps: forming afirst collector left vertical member in said substrate proximate to saidfirst base left diffused region and opposite from said first collectorright vertical member; forming a first buried collector right lobe insaid substrate, such that a bottom surface of said first collector rightvertical member contacts a top surface of said first buried collectorright lobe; and forming a first buried collector left lobe in saidsubstrate separate from said first buried collector right lobe, suchthat a bottom surface of said first collector left vertical membercontacts a top surface of said first buried collector left lobe; saidprocess of forming said base of said second bipolar transistor furtherincludes the step of forming a second base right diffused region in saidsubstrate proximate to said emitter of said second bipolar transistorand opposite from said second base left diffused region; and saidprocess of forming said collector of said second bipolar transistorfurther includes the steps: forming a second collector right verticalmember in said substrate proximate to said second base right diffusedregion and opposite from said second collector left vertical member;forming a second buried collector left lobe in said substrate, such thata bottom surface of said second collector left vertical member contactsa top surface of said second buried collector left lobe; and forming asecond buried collector right lobe in said substrate separate from saidsecond buried collector left lobe, such that a bottom surface of saidsecond collector right vertical member contacts a top surface of saidsecond buried collector right lobe.
 5. The process of claim 1, in which:said process of forming said collector of said first bipolar transistorincludes the steps: forming a first collector right vertical member insaid substrate proximate to said base of said first bipolar transistor;and forming a first collector right shallow well in said substrateextending to said top surface of said substrate, overlapping said firstcollector right vertical member, such that a lateral boundary of saidfirst collector right shallow well on a side closest to said base ofsaid first bipolar transistor extends past a corresponding lateralboundary of said first collector right vertical member; said process offorming said base of said first bipolar transistor includes the step offorming a first base right diffused region in said substrate betweensaid first collector right shallow well and said emitter of said firstbipolar transistor; said process of forming said breakdown inducingfeature of said first bipolar transistor is performed such that aidbreakdown inducing feature of said first bipolar transistor furtherincludes a region of said substrate between said first base rightdiffused region and said first collector right shallow well; saidprocess of forming said collector of said second bipolar transistorincludes the steps: forming a second collector left vertical member insaid substrate proximate to said base of said second bipolar transistor;and forming a second collector left shallow well in said substrateextending to said top surface of said substrate, overlapping said secondcollector left vertical member, such that a lateral boundary of saidsecond collector left shallow well on a side closest to said base ofsaid second bipolar transistor extends past a corresponding lateralboundary of said second collector left vertical member; said process offorming said base of said second bipolar transistor includes the step offorming a second base left diffused region formed in said substratebetween said second collector left shallow well and said emitter of saidsecond bipolar transistor; and said process of forming said breakdowninducing feature of said second bipolar transistor further is performedsuch that said breakdown inducing feature of said second bipolartransistor includes a region of said substrate between said second baseleft diffused region and said second collector left shallow well.
 6. Theprocess of claim 1, in which: said process of forming said base of saidfirst bipolar transistor includes the steps: forming a first base rightdiffused region in said substrate proximate to said emitter of saidfirst bipolar transistor; and forming a first base left diffused regionin said substrate proximate to said emitter of said first bipolartransistor and opposite from said first base right diffused region; saidprocess of forming said collector of said first bipolar transistorincludes the steps: forming a first collector right vertical member insaid substrate proximate to said first base right diffused region;forming a first collector left vertical member in said substrateproximate to said first base left diffused region and opposite from saidfirst collector right vertical member; forming a first buried collectorright lobe in said substrate, such that a bottom surface of said firstcollector right vertical member contacts a top surface of said firstburied collector right lobe; and forming a first buried collector leftlobe in said substrate separate from said first buried collector rightlobe, such that a bottom surface of said first collector left verticalmember contacts a top surface of said first buried collector left lobe;said process of forming said breakdown inducing feature of said firstbipolar transistor is performed such that said breakdown inducingfeature of said first bipolar transistor further includes a region ofsaid substrate between said first base right diffused region and saidfirst buried collector right lobe; said process of forming said base ofsaid second bipolar transistor includes the steps: forming a second baseleft diffused region in said substrate proximate to said emitter of saidsecond bipolar transistor; and forming a second base right diffusedregion in said substrate proximate to said emitter of said secondbipolar transistor and opposite from said second base left diffusedregion; said process of forming said collector of said second bipolartransistor includes the steps: forming a second collector left verticalmember in said substrate proximate to said second base left diffusedregion; forming a second collector right vertical member in saidsubstrate proximate to said second base right diffused region andopposite from said second collector left vertical member; forming asecond buried collector left lobe in said substrate, such that a bottomsurface of said second collector left vertical member contacts a topsurface of said second buried collector left lobe; and forming a secondburied collector right lobe in said substrate separate from said secondburied collector left lobe, such that a bottom surface of said secondcollector right vertical member contacts a top surface of said secondburied collector right lobe; and said process of forming said breakdowninducing feature of said second bipolar transistor is performed suchthat said breakdown inducing feature of said second bipolar transistorfurther includes a region of said substrate between said second baseleft diffused region and said second buried collector left lobe.
 7. Theprocess of claim 1, in which: said process of forming said base of saidfirst bipolar transistor includes the steps: forming a first base rightdiffused region in said substrate proximate to said emitter of saidfirst bipolar transistor; and forming a first base left diffused regionin said substrate proximate to said emitter of said first bipolartransistor and opposite from said first base right diffused region; saidprocess of forming said collector of said first bipolar transistorincludes the steps: forming a first collector right vertical member insaid substrate proximate to said first base right diffused region; andforming a first collector left vertical member in said substrateproximate to said first base left diffused region and opposite from saidfirst collector right vertical member; said process of forming saidfirst bipolar transistor further includes the steps: forming a firsttransistor right field plate over said substrate between said first baseright diffused region and said first collector right vertical member;and forming a first transistor left field plate over said substratebetween said first base left diffused region and said first collectorleft vertical member; said process of forming said breakdown inducingfeature of said first bipolar transistor further is performed such thatsaid breakdown inducing feature of said first bipolar transistorincludes said first transistor right field plate and a region of saidsubstrate under said first transistor right field plate; said process offorming said base of said second bipolar transistor includes the steps:forming a second base left diffused region in said substrate proximateto said emitter of said second bipolar transistor; and forming a secondbase right diffused region in said substrate proximate to said emitterof said second bipolar transistor and opposite from said second baseleft diffused region; said process of forming said collector of saidsecond bipolar transistor includes the steps: forming a second collectorleft vertical member in said substrate proximate to said second baseleft diffused region; and forming a second collector right verticalmember in said substrate proximate to said second base right diffusedregion and opposite from said second collector left vertical member;said process of forming said second bipolar transistor further includesthe steps: forming a second transistor left field plate over saidsubstrate between said second base left diffused region and said secondcollector left vertical member; and forming a second transistor rightfield plate over said substrate between said second base right diffusedregion and said second collector right vertical member; and said processof forming said breakdown inducing feature of said second bipolartransistor is performed such that said breakdown inducing feature ofsaid second bipolar transistor further includes said second transistorleft field plate and a region of said substrate under said secondtransistor left field plate.
 8. The process of claim 1, in which: saidprocess of forming said base of said first bipolar transistor includesthe steps: forming a first base right diffused region in said substrateproximate to said emitter of said first bipolar transistor; and forminga first base left diffused region in said substrate proximate to saidemitter of said first bipolar transistor and opposite from said firstbase right diffused region; said process of forming said collector ofsaid first bipolar transistor includes the steps: forming a firstcollector right vertical member in said substrate proximate to saidfirst base right diffused region; and forming a first collector leftvertical member in said substrate proximate to said first base leftdiffused region and opposite from said first collector right verticalmember; said process of forming said first bipolar transistor furtherincludes the steps: forming a first transistor right implanted region insaid substrate between said first base right diffused region and saidfirst collector right vertical member; and forming a first transistorleft implanted region in said substrate between said first base leftdiffused region and said first collector left vertical member; saidprocess of forming said breakdown inducing feature of said first bipolartransistor is performed such that said breakdown inducing feature ofsaid first bipolar transistor further includes said first transistorright implanted region; said process of forming said base of said secondbipolar transistor includes the steps: forming a second base leftdiffused region in said substrate proximate to said emitter of saidsecond bipolar transistor; and forming a second base right diffusedregion in said substrate proximate to said emitter of said secondbipolar transistor and opposite from said second base left diffusedregion; said process of forming said collector of said second bipolartransistor includes the steps: forming a second collector left verticalmember in said substrate proximate to said second base left diffusedregion; and forming a second collector right vertical member in saidsubstrate proximate to said second base right diffused region andopposite from said second collector left vertical member; said processof forming said second bipolar transistor further includes the steps:forming a second transistor left implanted region in said substratebetween said second base left diffused region and said second collectorleft vertical member; and forming a second transistor right implantedregion in said substrate between said second base right diffused regionand said second collector right vertical member; and said process offorming said breakdown inducing feature of said second bipolartransistor is performed such that said breakdown inducing feature ofsaid second bipolar transistor further includes said second transistorleft implanted region.
 9. The process of claim 1, in which: said processof forming said collector of said first bipolar transistor is performedsuch that said collector of said first bipolar transistor is n-type;said process of forming said base of said first bipolar transistor isperformed such that said base of said first bipolar transistor isp-type; said process of forming said emitter of said first bipolartransistor is performed such that said emitter of said first bipolartransistor is n-type; said process of forming said collector of saidsecond bipolar transistor is performed such that said collector of saidsecond bipolar transistor is n-type; said process of forming said baseof said second bipolar transistor is performed such that said base ofsaid second bipolar transistor is p-type; and said process of formingsaid emitter of said second bipolar transistor is performed such thatsaid emitter of said second bipolar transistor is n-type.
 10. Theprocess of claim 1, in which: said process of forming said collector ofsaid first bipolar transistor is performed such that said collector ofsaid first bipolar transistor is p-type; said process of forming saidbase of said first bipolar transistor is performed such that said baseof said first bipolar transistor is n-type; said process of forming saidemitter of said first bipolar transistor is performed such that saidemitter of said first bipolar transistor is p-type; said process offorming said collector of said second bipolar transistor is performedsuch that said collector of said second bipolar transistor is p-type;said process of forming said base of said second bipolar transistor isperformed such that said base of said second bipolar transistor isn-type; and said process of forming said emitter of said second bipolartransistor is performed such that said emitter of said second bipolartransistor is p-type.